How To Train Your Dragon 2

In order to unlock this you need to hit a rider who has already fallen off his dragon with a fireball. You can either wait until a rider is falling off and try to quickly target by holding and shoot him or just keep shooting rapidly and this should unlock without too much trouble. Hitting a rider three times with fireballs will cause them to fall and locking on afterwards with a well timed shot should cause this pop. This popped for me without actually trying for it. You also could try using a power up like the pail to knock a rider off followed immediately by a locked on fireball.